Arc discharge lamp including means for cooling envelope surrounding an arc tube

ABSTRACT

The arc tube of an arc discharge device is closely surrounded by, but spaced from, an optically transmissive envelope. The space between the tube and the envelope is filled with a transparent, inert, heat-transfer fluid. Forced cooling of the exterior envelope wall permits the arc tube to be operated at a substantially uniform wall temperature, at a high efficiency and high-wall loading.

United States Patent John M. Lo

Newton;

Warren Calvin Gungle. Danvers; John F. Wayrnouth, Marblehead. all of Mass.

Apr. 1, 1969 Nov. 9, 1971 Sylvanla Electric Products Inc.

lnventors Appl. No. Filed Patented Assignee ARC DISCHARGE LAMP INCLUDING MEANS FOR COOLING ENVELOPE SURROUNDING AN ARC TUBE 10 Claims, 2 Drawing Figs.

U.S. Cl 313/22, 313/26. 313/184, 313/227 Int. Cl ..H01j 61/18, H0lj 61/52 Field 01 Search 313/22. 25. 26, 225, 184. 227

References Cited UNITED STATES PATENTS 9/1932 Bogdandy et a1.

Primary Examiner- Roy Lake Assistant Examiner- Palmer C. Demeo Anorneys- Norman .1. OMalley and James Theodosopoulos ABSTRACT: The are tube of an arc discharge device is closely surrounded by, but spaced from an optically transmissive envelope. The space between the tube and the envelope is filled with a transparent. Inert. heat-transfer fluid. Forced cooling of the exterior envelope wall permits the arc tube to be operated at a substantially uniform wall temperature. at a high efficiency and high-wall loading.

PATENTEBuuv 9 l9?! JOHN M. LO

W.CALV|N GUNGLE OHN F. WAYMOUTH NVENTORS AGENT ARC DISCHARGE LAMP INCLUDING MEANS FOR COOLING ENVELOPE SURROUNDING AN ARC TUBE BACKGROUND OF THE APPLICATION FIELD OF THE INVENTION This invention relates to arc discharge lamps and particu- BACKGROUND OF THE INVENTION Because of their long life and good efficiency, high-pressure mercury vapor lamps have been used commercially for many years, in spite of the bluish green light they emitted. In the past few years, however, metallic halides have been added to the arc tube fill of such lamps to improve their efficiency and to produce a white light. An example of such a device is the Metalarc lamp shown in U.S. Pat. No. 3,407,327, issued on Oct. 22, 1968, to Koury et al. and assigned to the same assignee as the instant application. Because of the improvement in luminous efficiency and color rendition, such lamps have become commercially successful.

In the operation of such lamps, the coolest portion of the arc tube wall must be maintained at a temperature sufficiently high to cause an effective amount of the metallic halides in the arc tube to be in the vapor state, generally at least 600 C. The coolest portion of the arc tube wall is generally at the ends thereof near the electrodes. However, the hottest portion of the arc tube wall, generally located near the center thereof, must not exceed a maximum safe temperature. This maximum temperature is dependent on the arc tube composition, design and fill. When the arc tube is constructed of quartz, the maximum temperature can be about l,200 C., above which the quartz may begin to soften. However, certain metallic halides in the tube fill may react undesirably with the quartz at temperatures above about 900 C., or 1,000 C., and may cause premature failure of the lamp by devitrification or fracture of the quartz tube.

Thus the wall temperature at which such quartz arc tubes can be safely operated throughout their useful life is in the range of about 600 to 1,000 C.

However, because of the construction of present metallic halide lamps where the arc tube is surrounded by a bulbous glass envelope considerably larger than the arc tube, there is generally an appreciable temperature variation throughout the arc tube. For example, in a particular 175-watt metallic vapor lamp operated horizontally, the minimum and maximum tube wall temperatures were 600 and 935 C., respectively. The minimum temperature was at one end of the tube near the electrode while the maximum temperature was at the uppermost portion of the tube at about the center thereof.

When the same lamp was operated vertically, the minimum temperature was 575 C. and occurred at the lower end of the arc tube near the electrode. The maximum temperature was 770 C. and occurred at the upper portion of the tube between the center and the end. The arc tube was 14.5 millimeters in diameter by 24 millimeters long and its surface area was l7.6 square centimeters. Thus, when operated at its rated 175 watts, the arc tube was loaded to 9.9 watts per square centimeter of surface area and yielded about 70 lumens per watt.

In the case of a similar but larger lamp, rated at 2,500 watts, the arc tube was 33 millimeters in diameter by 2l4-millimeters long and was loaded to 9.75 watts per square centimeter of surface area. The minimum and maximum arc tube wall temperatures, when the arc tube was operated horizontally, were 640 and 840 C. respectively.

In the lighting industry and especially in the fields of outdoor lighting and high-bay interior lighting, it is desirable to use metallic vapor lamps having a high wattage. Through the use of such lamps, the total number of lamps and associated equipment such as poles, fixtures, reflectors and ballasts, that are needed to attain a predetermined light level can be significantly less than when lower wattage lamps are used.

If, consequently, a high wattage lamp for this purpose, say, l0,000 watts, were designed to about the same loading factor of 9 to 10 watts per square centimeter, as in the examples of the 175 and 2,5000-watt lamps above, the required are tube length would be about 3 feet. The enclosing envelope and the associated fixture and reflector would also be of correspondingly large proportion and the whole assembly would be too massive to be economically practical.

If however, the arc tube of such a lamp could be electrically loaded to a greater degree, the size of the arc tube, and thus the related accessory equipment, could be proportionately reduced in size.

This application relates to a metallic halide discharge lamp that can be more heavily loaded than presently available commercial halide lamps; in addition, the arc tube of said lamp can be operated at a more uniform wall temperature than the arc tubes of said commercially available lamps.

Another advantage of the lamp of this invention is that its restrike time is significantly lower than that of present commercially available halide lamps. Restrike time refers to the length of time that a lamp, after it has been extinguished, must be permitted to cool before an arc can be restruck across its electrodes.

The voltage necessary to strike an arc in a hot arc tube is much higher than that for a cold arc tube, that is, one at about room temperature. Thus, after a lamp has been extinguished, it must be permitted to cool to a temperature at which the striking voltage of the lamp is less than the maximum voltage produced by the ballast transformer.

In some cases the restrike time can be as long as 10 or 15 minutes if, for example, the arc tube must be cooled below about or C. before the arc can be restruck. Such a relatively long delay in reestablishing illumination after, say, a momentary electrical power interruption can be undesirable. In lamps of this invention, the restrike time is quite short because of the fast rate at which the arc tube is cooled.

SUMMARY OF THE INVENTION A lamp according to this invention comprises an arc tube, the walls of which are substantially transparent to radiation, especially the visible radiation, of an arc discharge. Such a tube is preferably made of quartz, although other materials, such as high silicaglass or transparent alumina, may be satisfactory.

Disposed within the tube are the usual spaced-apart arc discharge electrodes, and, if desired, a starter electrode. Also disposed within the arc tube is a fill including mercury and a metallic halide (except fluoride). As mentioned above, the metallic halide must be maintained, during operation, at a sufficiently high temperature to be effective in improving the color and/or efficiency of the lamp. The fill can also include an easily ionizable gas, such as argon, for ease in starting the lamp and an alkali metal, such as sodium, to aid in stabilizing the arc.

The are tube is disposed within, and spaced from, a sealed envelope, the walls of which are substantially transparent to the visible radiation of the arc. Preferably, the envelope is made of quartz, although other materials, such as those mentioned above for the arc tube, may be satisfactory. Also, the hard or soft glasses, so called, may be satisfactory in those cases where, during operation, the walls of the envelope are maintained at a temperature safely below the softening point of the glass.

The are tube is preferably supported within the envelope by lead-in wires having sufficient rigidity for that purpose. Said lead-in wires also serve to conduct external electrical power to the electrodes. Preferably, also, the arc tube is positioned substantially centrally within the envelope in order to provide substantially uniform spacing between the walls of the arc tube and the walls of the envelope for the purpose of uniform cooling of the arc tube, as will be hereinafter described.

The space between the arc tube and the envelope is preferably small for high efficiency of heat transfer from the arc tube to the envelope. In addition, said space is filled with a fluid that is transparent to visible radiation, inert to the materials in contact therewith, especially at the high temperatures of operation, and that has adequate heat transfer properties for the purposes of this invention. Helium is an example of such a fluid. Nitrogen and argon are examples of other fluids that may be used in some cases, although their heat conductivities are substantially lower than that of helium. The named fluids do not react significantly with the commonly used metals comprising the iead-in wires or with the materials comprising the arc tube and envelope.

Other media, in addition to gases, may also be used as the heat-transfer fluid within the above-mentioned space if said media are transparent, suitably inert and have adequate heat conductivity.

In order to attain the heavy loading and uniform arc tube wall temperature of this invention, it is necessary that the external wall of the envelope be forcibly cooled. Mere exposure of said wall to a nonflowing medium or a medium circulating solely due to convection currents is usually insufficient to attain the desired results. Furthermore, said wall must be cooled at a sufficient rate to obtain a substantially uniform arc tube wall temperature.

The amount of temperature uniformity necessary for this invention can vary between different lamps, and is dependent on, among other things, the composition of the arc tube fill, the arc-tube-operating temperature and the spacing of the electrodes from the arc tube wall. However, the uniformity of arc tube wall temperature is superior in lamps of this invention than in presently available commercial metallic halide lamps. An illustration of this superiority can be shown by a comparison of the distribution of the fill on the arc tube walls.

As mentioned above, the fill comprises mercury and a metallic halide and also, generally, an easily ionizable gas and an alkali metal, usually as a halide. In order for a lamp to operate as designed, that is, with high efficiency and/or white light emission, an effective amount of the desired metallic components must be present in the arc stream.

In order to enter the arc stream, said components must first be vaporized. Mercury is a liquid at room temperature and boils at 355 C. at atmospheric conditions. Since the arc tube operates at a considerably higher temperature, all the mercury is vaporized; therefore, the vapor pressure of the mercury is controlled by the total amount thereof in the fill and generally does not exceed about 5 to atmospheres.

However, the metallic halides are generally solids at room temperature and have much lower vapor pressures than mercury. Therefore, they must be heated to a high enough temperature, so that an effective amount thereof is vaporized. If two or more such metallic halides are present in the till, the desired radiation is obtained by carefully controlling the amounts of the halides having the higher vapor pressures and adding an excess of the halide having the lowest vapor pressure.

At the usual arc-tube-operating temperatures, the metallic halides are generally in the liquid state, in addition to that amount that is vaporized. The distribution pattern of the balides, as liquids, on the interior wall of the arc tube can be readily observed. In the prior art lamps, the liquid haiides were observed only at the coolest portion of the arc tube and were distributed over only a small fractional part of the entire arc tube wall. Such a pattern indicated a nonuniform arc tube wall temperature, typical of arc tubes the temperature variations of which are shown in Description of the Prior Art.

However, in arc tubes in accordance with this invention, the temperature variation throughout the arc tube wall, during operation, is considerably less and the liquid halides are distributed over a much greater proportion of the interior wall. At optimum conditions of arc tube construction, wall loading,

. envelope spacing and rate of cooling, the temperature of the arc tube wall is uniform enough so that the liquid halides are distributed over substantially the entire interior wall.

Upon cooling of the arc tubes, the halides solidify generally in place and remain visible. Thus, the pattern of temperature variation may also be observed on the cooled tube. The mercury, of course, is visible as a drop of liquid at the lowest portion of the tube.

In a preferred construction, the space between the arc tube and the envelope is small to improve the rate of heat transfer from the wall of the arc tube through the inert fluid to the wall of the envelope. Preferably also the space is substantially uniform in dimension at least around the arc length portion of the arc tube since the maximum wall temperature generally occurs thereat.

The envelope may be forcibly cooled by flowing a suitable gas or liquid therearound at the desired rate. Preferably, however, the envelope is disposed within a suitable transparent container having an inlet and an outlet to permit cooling fluid (liquid or gas) from an external source to be flowed around the envelope. Alternatively the lamp and coolant may comprise a closed system wherein the coolant, after having been heated by the envelope, is itself cooled apart from the lamp and recirculated back to the envelope to provide continuous cooling.

In another method of forcible cooling of the envelope, the envelope may be partially or completely immersed in a suitable evaporatable liquid, such as water. The heat from the envelope would cause the liquid to evaporate; the vapors could be condensed above the lamp and the condensate could be caused to flow back either directly or over the envelope of the main body of the liquid. In such a case, the exterior wall of the envelope would be maintained at substantially the same temperature as the boiling temperature of the liquid.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a drawing of one embodiment of a lamp according to this invention wherein an arc tube is disposed within an envelope which, in turn, is disposed within a transparent container having an inlet and an outlet.

FIG. 2 is a drawing of another embodiment in which only part of the envelope is disposed within a transparent container.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, are tube 1 was made of quartz and was cylindrically shaped. Arc tube 1 was sealed at each end by press seals 2. Disposed within arc tube 1 were discharge electrodes 3 and starter electrode 4. Also disposed within arc tube 1 was till 5 comprising mercury and metallic halides. Electrodes 3 and 4 were connected to the usual foil connectors 6 disposed within press seals 2. Extending beyond press seals 2 were lead-in wires 7, one end of each of which was connected to a foil connector 6 and embedded in press seal 2.

Substantially uniformly surrounding, but spaced from, are tube l was sealed quartz envelope 8 having press seals 9 at each end thereof.

Arc tube 1 was supported within envelope 8 by rigid lead-in wires 7, the extending ends of which were embedded in press seals 9. Said ends were also electrically connected to foil connectors l0 embedded within press seals 9. Extending beyond press seals 9 were lead-in wires 11, one end of each of which was connected to a foil connector 10 and embedded in press seal 9.

Shown in phantom is container 12 in accordance with one embodiment of this invention. Container 12 has an inlet 13 and an outlet 14 and can be made of hard or soft glass. Lead-in wires ll sealably extend through the walls of container 12 to provide for electrical connection to an external power source. Envelope 8 can be supported within container 12 by lead-in wires 11 or, if desired, an additional supporting structure may be used such as an assembly clamped to the ends of envelope 8 and supported against the inner walls of container 12.

In FIG. 2, are tube I and envelope 8 are similar to those of FIG. I and the same identifying numbers are used. However, envelope 8 in FIG 2 is proportionately longer than that of FIG 1 and metallic spacers 16 on lead-in wires 7 provide additional support for arc tube 1. Specifically, spacers 16 are formed of resilient metal ribbons in the general shape of a cross. The ends of spacers 16 bear against the inner wall of envelope 8 and spacers 16 are fixed to lead-in wires 7 at a point intermediate press seal 2 and press seal 9, although somewhat nearer press seal 9. Thus spacers 16 substantially prevent any transverse movement of arc tube 1 within envelope 8.

Container 12, in FIG. 2, is made of glass and its ends are fused directly to envelope 8 at points between press seal 2 and press seal 9. When a suitable coolant is circulated through container 12 by means of inlet 13 and outlet 14, only the surrounded portion of envelope 8 is directly cooled thereby. However, the length of the surrounded portion of envelope 8 exceeds the length of arc tube 1 and, therefore, are tube 1 is substantially uniformly cooled thereby.

Sealed tipoff 15 is the remains of an exhaust tube fastened to the wall of envelope 8 which permitted envelope 8 to be filled with helium.

In an example of the heavier wall-loading obtainable by a lamp in accordance with this invention, the arc tube from a commercial 400-watt lamp was used. Arc tube 1 had an outside diameter of 22 millimeters, a wall thickness of l millimeter, an arc length of 45 millimeters and a wall surface of. 46 square centimeters. The fill consisted of 70 milligrams of mercury, milligrams of sodium iodide, 5.5 milligrams of scandiurn iodide, 2 milligrams of cesium iodide, and 25 torr of argon. When operated at 400 watts in its prior art construction, the wall loading was 8.6 watts per square centimeter and the temperature of the arc tube wall varied from 650 to 860 C.

Operation at wattages in excess of 400 watts would have caused the maximum wall temperature to undesirably approach or exceed the maximum temperature, for the particular arc tube, of 900 to l,000 C.

However, in accordance with this invention, arc tube 1 of the commercial 400-watt lamp was enclosed in envelope 8 the dimensions of which were 25 millimeters inside diameter, 1- millimeter wall thickness and 150-millimeters long. The space between arc tube 1 and envelope 8 was l rmillimeters across and was filled with helium at a room temperature pressure of 500 torr. Envelope 8 was cooled by a fast moving current of air blown longitudinally across the outer wall thereof while arc tube 1 was operated at 1,750 watts, equivalent to a loading of 42 watts per square centimeter. Under these conditions, the arc tube wall was at a substantially uniform temperature of about 800 C., and the metallic iodides were distributed in liquid form over the inner arc tube wall for substantially the entire arc length. The melting points of sodium iodide, scandium iodide and cesium iodide are 662 C., 964' C., and 621 C. respectively. Their vapor pressure at 800C. are 2 millimeters, 45 millimeters and 1.1 millimeters, respectively.

At normal operation, the 400-watt lamp had an efficiency of 85-lumens per watt, whereas, when operated at 1,500 watts according to this invention, the efficiency increased to 95 lu, mens per watt. This may be attributable to the fact that the electrical power in such devices is consumed in electrode losses and radiation losses (approximately one-third of which is heat radiation). For a particular are tube, the electrode losses remain fairly constant above a given loading level. Thus, increased loading above this level results in a higher percentage of the electrical power being converted to radiant energy.

This last example illustrates that a container need not be an integral feature of this invention since forced cooling may be effected without such a container. Furthermore, if a container is desired, it need not be a completely enclosing container such as is shown in FIG. 1. It could be, as in FIG. 2, a glass tube encircling envelope 8 but sealed thereto at or near the ends thereof. In such a case the space therebetween, filled with heat-transfer fluid, should extend at least as far as the ends of arc tube 1 or, preferably, therebeyond.

As mentioned previously, the space or gap between arc tube 1 and envelope 8 is preferably small for the purpose of good efl'iciency of heat transfer from the arc tube to the envelope.

However, in any particular lamp, the optimum gap is dependent on, among other things, the arc tube wall loading of the lamp and the thermal conductivity of the heat-transfer fluid.

For example, in a particular arc tube normally operated at 400 watts, it was desired to maintain the inner arc tube wall temperature at about 950 C. while envelope 8 was cooled by boiling water. Thus, the temperature drop from the inner wall of arc tube 1 to the outer surface of envelope was about 850 C.

When the gap between arc tube 1 and envelope 8 was 1 millimeter and was filled with helium, it was found that the temperature parameters listed above could be met when the input power to the arc tube was 45 watts per square centimeter. However, if the gap were filled with nitrogen, only 7.5 watts of input power per square centimeter is needed to obtain the 850 C. temperature differential, the reason being that nitrogen has lower thennal conductivity than helium. The thermal conductivity coefficient for helium is 0.000339 gram calories per square centimeter per second per degree centigrade per centimeter while for nitrogen it is 0.00005 24.

However, if the nitrogen-filled gap were reduced to 0.l millimeter, the arc tube could be operated at 62.5 watts per square centimeter in order to maintain the 850 C. temperature difierential. Thus, the gap thickness can be varied, as desired, depending on the thermal conductivity of the heat transfer fluid used. In the case of argon, which has lower thermal conductivity even than nitrogen, the gap would have to be even less than that above for nitrogen, if it were desired to load the arc tube to the same extent. However, there the desired maximum load is less, the gap can be increased proportionately.

Of course, theheat-transfer fluid may be a blend of more than one individual fluid.

The fill pressure of the heat-transfer fluid does not appear to be especially significant in affecting the rate of heat transfer from are tube 1 to envelope 8. It was found, in a particular lamp utilizing helium as the heat-transfer fluid, that at fill pressures of 50 and 500 ton and at wall loadings of about 45 watts per square centimeter, the rate of heat dissipation from the outer surface of envelope 8 was about equal.

In-another embodiment of this invention, the ends of arc tube 1, shown press sealed in the drawings, may be sealed directly to the ends of envelope 8. In such a case, the lead-in wires would be embedded in the common seal area. Also, in such a case, the space between arc tube 1 and envelope 8, which is filled with heat-transfer fluid, would have to extend at least the arc length portion of arc tube 1.

We claim:

I. An arc discharge lamp comprising: an arc tube containing a fill including metallic halide; a transparent envelope surrounding but spaced from said are tube for at least the arc length portion of said tube, the thickness of said space being substantially uniform for a least the length of said portion; a transparent, inert, heat-transfer fluid within said space; means for forcibly cooling the exterior wall of said envelope.

2. The lamp of claim 1 wherein said arc tube is supported within said envelope by substantially rigid lead-in wires sealed into the ends of said are tube.

3. The lamp of claim 1 wherein the ends of said arc tube are joined to said envelope.

4. The lamp of claim 1 wherein said inert fluid is helium.

5. The lamp of claim 1 wherein said envelope is enclosed within a transparent container.

6. The lamp of claim 5 wherein said container has an inlet and an outlet for a coolant.

7. An arc discharge device comprising: an arc tube having press seals at each end and containing a fill including metallic halide; electrodes disposed within said arc tube and connected to foil connectors embedded within said press seals; a sealed transparent envelope concentrically surrounding said arc tube but spaced therefrom, said envelope extending beyond the ends of said are tube; press seals at each end of said envelope and foil connectors embedded within said press seals, the foil at each end and containing a fill including metallic halide; electrodes disposed within said arc tube and connected to connectors embedded within said sealed ends, a transparent envelope concentrically surrounding, but spaced from, said are tube for at least the arc length portion thereof, said envelope being sealed to the sealed ends of said arc tube; a transparent, inert, heat-transfer fluid disposed within the space between said are tube and said envelope; means for forcibly cooling the exterior wall of said envelope.

# t I t 

1. An arc discharge lamp comprising: an arc tube containing a fill including metallic halide; a transparent envelope surrounding but spaced from said arc tube for at least the arc length portion of said tube, the thickness of said space being substantially uniform for at least the length of said portion; a transparent, inert, heat-transfer fluid within said space; means for forcibly cooling the exterior wall of said envelope.
 2. The lamp of claim 1 wherein said arc tube is supported within said envelope by substantially rigid lead-in wires sealed into the ends of said arc tube.
 3. The lamp of claim 1 wherein the ends of said arc tube are joined to said envelope.
 4. The lamp of claim 1 wherein said inert fluid is helium.
 5. The lamp of claim 1 wherein said envelope is enclosed within a transparent container.
 6. The lamp of claim 5 wherein said container has an inlet and an outlet for a coolant.
 7. An arc discharge device comprising: an arc tube having press seals at each end and containing a fill including metallic halide; electrodes disposed within said arc tube and connected to foil connectors embedded within said press seals; a sealed transparent envelope concentrically surrounding said arc tube but spaced therefrom, said envelope extending beyond the ends of said arc tube; press seals at each end of said envelope and foil connectors embedded within said press seals, the foil connectors of said envelope being electrically connected to the foil connectors of said arc tube; a transparent, inert, heat-transfer fluid disposed within the space between said arc tube and said envelope; means for forcibly cooling the exterior wall of said envelope.
 8. The device of claim 7 wherein said envelope is enclosed with a transparent container.
 9. The device of claim 7 wherein at least the arc length portion of said envelope is enclosed within a transparent container.
 10. An arc discharge device comprising: an arc tube sealed at each end and containing a fill including metallic halide; electrodes disposed within said arc tube and connected to connectors embedded within said sealed ends; a transparent envelope concentrically surrounding, but spaced from, said arc tube for at least the arc length portion thereof, said envelope being sealed to the sealed ends of said arc tube; a transparent, inert, heat-transfer fluid disposed within the space between said arc tube and said envelope; means for forcibly cooling the exterior wall of said envelope. 